Spend good day in Southern Utah (5,3)
I believe the answer is:
shell out
'spend' is the definition.
(I've seen this before)
'good day in southern utah' is the wordplay.
'good day' becomes 'hello' ('good day' is a way of saying hello).
'in' indicates putting letters inside.
'southern' becomes 's' (abbreviation).
'utah' becomes 'ut' (I've seen this in other clues).
's'+'ut'='sut'
'hello' going into 'sut' is 'SHELL OUT'.
(Other definitions for shell out that I've seen before include "gimme money!" , "pay" , "Come up with ready" , "Spend" , "Hand over money" .)